This invention relates in general to catalytic apparatus for treating gases and, in particular, to improved structures for mounting actively coated monolithic carrier material in a housing.
As is well known, purification of exhaust gases from internal combustion engines, particularly in motor vehicles, is today generally achieved by afterburning. This afterburning may be hastened by feeding the exhaust gases in such a system through catalysts consisting of a carrier which is coated with a catalytic substance that effects a chemical conversion of the noxious gas constituents. In such a system, the carrier may either be a loose material or a monolith with channels passing through it. In the automotive field, catalysts with monolithic carriers have proved particularly advantageous in this connection. These carriers are housed in gas-tight, sheet-metal or cast-metal, heat-resistant housings whereby they can be connected to the exhaust system of the internal combustion engine. However, when the monolithic carrier is mounted in the casing, several problems arise as a result of the fraility of the carrier material.
Carriers that currently are employed, consist of a porous ceramic material having limited mechanical strength. Thus, it is not possible to exert clamping forces on them with sufficient strength to assure a secure, solid mounting of the carriers. On the other hand, the catalyst must be arranged in the immediate vicinity of the internal combustion engine so that its start-up time, following the starting of the engine, will be short. Accordingly, the vibrations emanating from the internal combustion engine are transmitted to the catalyst housing and these vibrations can damage the carrier structure. In order to prevent such damage to the carrier, the carrier must be mounted flexibly or elastically. A mounting of this sort also serves the need to accommodate the relatively large cross-sectional tolerances of the carrier which occur during the manufacturing process and the differences in thermal expansion that result from the different coefficients of thermal expansion of the carrier material and the metal housing.
In one known catalytic apparatus (German Provisional Pat. No. 1,476,507, U.S. Pat. No. 3,441,381 to Keith et al.), the carrier is held in a cylindrical housing having, in the annular space between the housing and the carrier body, a springy corrugated jacket preferably made of wire mesh, which tightly surrounds the carrier body. It has been found, however, that such an intermediate layer consisting of a springy wire mesh does not solve the above-described mounting problems as the high temperature in the catalyst causes the wire mesh to burn out. If this wire mesh were made of a material that would not be affected by high temperatures, a catalyst of this type, which is intended for mass use, would be far too expensive.
These and other shortcomings of the prior art are overcome by the present invention.